In a keyboard or other data input device, variable capacitance push-button switches (hereinbelow termed "capacitance type switches") have come into wide use. In such switches, actuation of the push-button changes the capacitance of the switch. In a keyboard, for example, the changed capacitance of a depressed switch is detected and its row and column determined, causing to be generated a binary signal uniquely associated with the depression of that particular switch. Thus, the difference between the two capacitive levels of the switch needs to be made sufficiently great, and the capacitance level at each of the two switch positions must remain uniform throughout the life of the push-button switch and under a variety of pressures applied to the push-button switch by an operator's finger. Typically, capacitance type switches achieve two levels of capacitance by, upon actuation of the push-button, effectively separating or bringing together two conductive plates separated by a dielectric in order to change the distance between the plates and, consequently, the capacitance between the plates.
Capacitance is calculated using the equation: EQU C=AK.epsilon..sub.0 /t,
where
C is the capacitance
.epsilon..sub.0 is the permittivity of empty space (8.85.times.10.sup.-2 coul.sup.2 /newton.sup.2 -m.sup.2)
K is the dielectric constant
A is the plate area, and
t is the dielectric thickness.
As seen, by decreasing the dielectric thickness t between the plates, the capacitance is increased.
FIG. 1 is a sectional view of the essential portions of a known capacitance type switch, and principally illustrates the electrical construction of the switch. As shown in FIG. 1, a push rod 31 carries a movable electrode 36 having a dielectric film 35 thereover. The electrode 36 is movable relative to a pair of stationary electrodes 33 and 34 disposed on an insulating substrate 32. One of the stationary electrodes, such as electrode 33, serves as an input and the other electrode serves as the output of the switch. In FIG. 1, let d.sub.a and d.sub.f denote the thicknesses of the air gap and the dielectric film 35, respectively. When the push-button is pressed and dielectric film 35 approaches and contacts stationary electrodes 33 and 34, the air gap d.sub.a is reduced and the capacitance is thereby increased. Capacitance of the switch is determined by the series capacitance of the capacitor formed by stationary contact 33 and movable electrode 36 and the capacitor formed by stationary electrode 34 and movable electrode 36. Two patents which utilize this general means of providing two capacitance levels by actuation of a push-button switch are U.S. Pat. Nos. 3,965,399 to Walker, Jr. et al. and 4,423,464 to Tamura et al. These prior art patents concern themselves with increasing the reliability of the switch by obtaining a consistent dielectric thickness between the capacitive plates in both switch positions so that the two capacitance levels remain uniform throughout the life of the switch. As seen by inspection of FIG. 1, the dielectric thickness of, for example, dielectric 35 in FIG. 1 as well as the mechanical movement of the switch, controlling the air gap, must be precisely controlled to obtain uniform capacitances in both switch positions. These types of switches are therefore manufactured with high precision, resulting in a concomitant increased cost of manufacture. Further, the switch must be depressed to its full extent to achieve the full high capacitance level, adding to the complexity of the manufacture of the switch since the dielectric material must not be damaged by contact with the stationary electrodes.